Fix OO operator protocols (NotImplemented, type checks, hash)

[endolith]

Summary

This addresses object-oriented / dunder-method issues in just_intonation.py: correct use of NotImplemented, type checks before touching the RHS, removal of dead state that caused misleading equality, and __hash__ where __eq__ is defined.

Issues fixed

1. Interval.__add__ / __sub__
   When the right-hand side was not an Interval, these methods fell off the end and returned None, which breaks the numeric protocol and can surface as confusing TypeErrors. They now return NotImplemented.

2. Interval rich comparisons and __eq__
__lt__ / __gt__ / __le__ / __ge__ called _F(b) for arbitrary b, which could AttributeError (e.g. Interval(2, 1) < 2). They now require isinstance(b, Interval) and otherwise return NotImplemented.
   __eq__ now returns NotImplemented for non-Interval operands so reflexive equality is handled correctly instead of blindly returning False.

3. Interval._terms
   It was only set on one constructor path, was never read anywhere in the repo, and happened to make Chord(M3) == Interval(5, 4) true in tests by accidental tuple equality. That assignment was removed so Interval does not carry misleading private state.

4. Pitch.__init__
   After normalizing frequency, integral values were stored with int(frequency) (the original argument) instead of the coerced value. It now uses int(self._frequency), which is correct when frequency is already a Pitch or other wrapped type.

5. Pitch ordering and __eq__
   Same pattern as Interval: isinstance(b, Pitch) and NotImplemented instead of b.frequency on arbitrary objects.

6. Pitch.__hash__
   With __eq__ overridden, defining __hash__ keeps hashing behavior explicit and consistent with equality (based on stored frequency).

7. Chord
class Chord(): → class Chord:. __eq__ now checks isinstance(b, Chord) and returns NotImplemented otherwise (avoids AttributeError on b._terms). __hash__ added so chords can be used in sets/dicts like intervals.

Test change

• Chord(M3) == Interval(5, 4) was only true because of the removed Interval._terms quirk. The test now asserts Chord(M3) != Interval(5, 4), which matches type-safe equality.

Verification

• pytest (full suite): 21 passed
• pytest --doctest-modules just_intonation.py: 17 passed
• flake8 (syntax/undefined-name select on touched files): clean